Fuel cells directly convert chemical energy into electricity thereby eliminating the mechanical process steps that limit thermodynamic efficiency, and have been proposed as a power source for many applications. The fuel cell can be two to three times as efficient as the internal combustion engine with little, if any, emission of primary pollutants such as carbon monoxide, hydrocarbons and NOx. Fuel cell-powered vehicles which reform hydrocarbons to power the fuel cell generate less carbon dioxide (green house gas) and have enhanced fuel efficiency.
Fuel cells, including PEM fuel cells [also called solid polymer electrolyte or (SPE) fuel cells], generate electrical power in a chemical reaction between a reducing agent (hydrogen) and an oxidizing agent (oxygen) which are fed to the fuel cells. A PEM fuel cell includes an anode and a cathode separated by a membrane which is usually an ion exchange resin membrane. The anode and cathode electrodes are typically constructed from finely divided carbon particles, catalytic particles supported on the carbon particles and proton conductive resin intermingled with the catalytic and carbon particles. In typical PEM fuel cell operation, hydrogen gas is electrolytically oxidized to hydrogen ions at the anode composed of platinum reaction catalysts deposited on a conductive carbon electrode. The protons pass through the ion exchange resin membrane, which can be a fluoropolymer of sulfonic acid called a proton exchange membrane. H2O is produced when protons then combine with oxygen that has been electrolytically reduced at the cathode. The electrons flow through an external circuit in this process to do work, creating an electrical potential across the electrodes.
Fuel processors (also known as fuel reformers) generate a hydrogen-containing gas stream which can be used to supply the fuel cell or generate hydrogen for specialty chemical application or for storage in a hydrogen fuelling station. Fuel processors include reactors that steam reform hydrocarbon feedstocks (e.g., natural gas, LPG) and hydrocarbon derivatives (e.g., alcohols) to produce a process stream enriched in hydrogen. Another viable process for reforming hydrocarbons is the so-called autothermal reforming process, which combines partial oxidation with steam reforming. Other by-products from the reforming of hydrocarbon include carbon monoxide and carbon dioxide. For example, methane is converted to hydrogen, carbon monoxide and carbon dioxide by the three reactions below:CH4+H2O→3H2+COCH4+2H2O→4H2+CO2CH4+½O2→2H2+CO
The resulting gas is then reacted in the water-gas shift reactor where the process stream is further enriched in hydrogen by reaction of carbon monoxide with steam in the water-gas shift (WGS) reaction:CO+H2OCO2+H2
In fuel processors, the reaction is often conducted in two stages for purposes of heat management and to minimize the outlet CO concentration. The first of two stages is optimized for reaction at higher temperatures (about 350° C.) and is typically conducted using catalysts based on combinations of iron oxide with chromia. The second stage is conducted at lower temperatures (about 200° C.) and is typically conducted using catalysts based on mixtures of copper and zinc materials.
Other catalysts that can be used to conduct the water-gas shift reaction include platinum (Pt)-based catalysts such as platinum on an alumina support or platinum on a cerium oxide containing support. While effective at producing hydrogen using the water-gas shift reaction when operated at temperatures above about 300° C., water-gas shift reaction catalysts also cause the formation of methane (CH4) by catalyzing the reaction of CO with hydrogen as shown below:CO+3H2→CH4+H2O.
This undesired side reaction sacrifices three moles of hydrogen for each mole of carbon monoxide converted to methane. Methanation can also occur under these conditions with carbon dioxide according to the equation shown below:CO2+4H2→CH4+2H2O
In this side reaction, four moles of hydrogen are consumed for each mole of carbon dioxide converted to methane. The production of methane during the water-gas shift reaction (referred to herein as “methanation”) is a side reaction that consumes hydrogen gas in an exothermic reaction to ultimately reduce the hydrogen yield from the water gas shift reaction. Moreover, the methanation reactions accelerate with increasing catalyst bed temperatures. This property presents a liability, as the exothermic reaction can result in a runaway reaction with carbon dioxide, in addition to carbon monoxide, being methanated. Major hydrogen loss can occur and the catalyst can be damaged by high temperatures. In addition, methane is a greenhouse gas. The fuel cell is advertised as an emission-free energy producer, and release of methane is undesirable. Methane is difficult to combust during normal operating conditions of the fuel cell, so producing an appreciable quantity of methane is environmentally unfavorable.
Pt based catalysts are now the best option for the displacement of base metal catalysts in residential fuel processors and in hydrogen generators for on-site hydrogen generation and low temperature PEM fuel cells. However, aging of the Pt based catalysts is a known disadvantage for this type of catalyst. Because of the aging problems Pt catalysts are widely regarded as unstable in various operations.
Metals such as cobalt (Co), ruthenium (Ru), palladium (Pd), rhodium (Rh) and nickel (Ni) have also been used as WGS catalysts but are normally not too active for the selective WGS reaction and cause methanation of CO to CH4 under typical reaction conditions. In other words, the hydrogen produced by the water gas shift reaction is consumed as it reacts with the CO feed in the presence of such catalysts to yield methane. This methanation reaction activity has limited the utility of metals such as Co, Ru, Pd, Rh and Ni as water gas shift catalysts.
Pt—Re bimetallic catalysts for use in the WGS reaction have recently been suggested. For example, Pt—Re on a ceria-zirconia support was shown to enhance the WGS rate compared with the rate observed with Pt alone supported on ceria-zirconia, “Pt—Re bimetallic supported on CeO2—ZrO2 mixed oxides as water-gas shift catalyst”, Choung et al., Catalysis Today 99 (2005) 257-262. U.S. Pat. No. 6,777,117 issued Aug. 17, 2004 and U.S. Pat. Pub. No. 2003/0186804 published Oct. 2, 2003 disclose similar Pt—Re WGS catalysts.
A need exists, for operating a Pt—Re bimetallic based water gas shift catalyst under conditions that reduce the aging process of such catalyst and provide a stable condition for the production of effective amounts of hydrogen for use in fuel cells.